1. Field of the Invention
The present invention relates to an n-type modulation-doped multi quantum well semiconductor laser device capable of emitting light a wavelength band of 600 to 1,650 nm, and more specifically, to an n-type modulation-doped multi quantum well semiconductor laser device capable of lasing with higher output power than a conventional semiconductor laser device.
2. Prior Art
In a multi quantum well semiconductor laser device, which can carry out high-output oscillation, an active layer has a multi quantum well (hereinafter referred to as MQW) structure, an alternate hetero-junction structure including well layers and barrier layers. Each well layer has a nanometer thickness and is formed of a semiconductor material with low band gap energy. Each barrier has a nanometer thickness and is formed of another semiconductor material with higher band gap energy than that of each well layer. The semiconductor layer device of this type can oscillate at lower threshold current than a semiconductor laser device that has a bulk active layer.
The multi quantum well semiconductor laser device has a layer structure such that a lower clad layer, a lower optical confinement layer, and the active layer are successively formed on a semiconductor substrate by an epitaxial crystal growth method, and an upper optical confinement layer, an upper clad layer, and a contact layer are successively formed on the active layer.
This layer structure is subjected to cleavage such that a predetermined cavity length is obtained. A single-layer anti-reflection film of, e.g., SiNx is formed on one cleaved surface (front facet) that serves as an emission end face, and a high-reflection film including alternate layers of, e.g., xcex1-Si and SiNx is formed on the other cleaved surface (rear facet). Further, an n-type electrode of, e.g., Auxe2x80x94Ge/Ni/Au is formed on the back surface of the semiconductor substrate, and a p-type electrode of, e.g., Ti/Pt/Au is formed on the contact layer with an ohmic junction.
The MQW semiconductor laser device of this type oscillates with higher output power than a semiconductor laser device that has a bulk active layer. However, oscillation with still higher output power is required in the case where the MQW semiconductor laser device is used as a pumping laser device for exciting an Er-doped fiber amplifier (EDFA), for example.
FIG. 1 is a energy band diagram of conduction band showing a layer structure A near the active layer of a high-output MQW semiconductor laser device that oscillates in a wavelength band of 1,300 nm.
The layer structure A is formed between an n-type lower clad layer 1a of n-type InP and a p-type upper clad layer 1b of p-type InP. In this structure A, an active layer having three quantum wells is interposed between lower and upper optical confinement layers 2a and 2b of GaInAsP having a thickness of 120 nm and a band gap wavelength (xcexg) of 1.1 xcexcm each.
The active layer is composed of well layers 3a of undoped InAsP having a thickness of 8 nm and compression strain of 1.45% and barrier layers 3b of undoped InGaAsP formed between the well layers and having a thickness of 10 nm and a band gap wavelength (xcexcg) of 1.1 xcexcm each.
Uomi theoretically predicted that the laser performance of this MQW structure can be improved by selectively modulation-doping the barrier layers with an n-type dopant without changing the undoped state of the well layers (K. Uomi; Japanese Journal of Applied Physics, vol. 29, No. 1, January 1990, see pp. 81-87).
If the barrier layers are modulation-doped with the n-type dopant, according to this Uomi""s theoretical prediction, the necessary injected carrier density for transparency is much lower than in the case where the barrier layers are not modulation-doped (undoped).
In the case of a laser device having the n-type modulation-doped active layer, therefore, the threshold current can be made lower than in the case of a laser device having an undoped MQW structure if a high-reflection film is formed on each facet of the device to lower mirror loss. In the case of an n-type modulation-doped MQW semiconductor laser device proposed by Uomi, however, the cavity length is as short as 200 xcexcm or less, and the threshold current is expected to be adjusted to about 1 mA or less by forming a high-reflection film on each facet of the device.
An object of the present invention is to provide an n-type modulation-doped MQW semiconductor laser device, capable of oscillating with higher output power than a conventional n-type modulation-doped MQW semiconductor laser device by controlling various parameters.
Another object of the invention is to provide an n-type modulation-doped MQW semiconductor laser device, capable of emitting light with various wavelengths in a wavelength band of 600 to 1,650 nm by selecting a semiconductor material for well layers.
In order to achieve the above objects, according to the present invention, there is provided an n-type modulation-doped multi quantum well semiconductor laser device having a multi quantum well structure composed of a hetero-junction structure including well layers and barrier layers, characterized in that:
each of the well layers and each of the barrier layers are formed of an undoped semiconductor material and a semiconductor material modulation-doped with an n-type dopant, respectively;
a low-reflection film and a high-reflection film are formed on the front and rear faces, respectively;
the cavity length is not shorter than 800 xcexcm; and
mirror loss (xcex1m) given by
xcex1m=(1/2L)ln{1/(Rfxc3x97Rr)},
where L, Rf and Rr are the cavity length (cm), reflectance of the front facet, and reflectance of the rear face, respectively, is not higher than 15 cmxe2x88x921.
More specifically, in the n-type modulation-doped multi quantum well semiconductor laser device of the invention, the doping concentration of the n-type dopant in the barrier layers ranges from 5xc3x971017 to 3xc3x971018 cmxe2x88x923, and the reflectance (Rf) of the front facet ranges from 1 to 10% and the reflectance (Rr) of the rear facet from 80 to 100%.
In the laser device of the invention, moreover, MQW structure may be composed of a strain-compensating type well layer, the n-type dopant is Si, Se or Sn, preferably, and the well layer or layers are 1 to 15 in number preferably.
Furthermore, according to the present invention, there is provided an n-type modulation-doped multi quantum well semiconductor laser device emitting light in a wavelength band of 1200 to 1650 nm by forming each of the well layers of InAsP, emitting light in a wavelength band of 900 to 1650 nm by forming each of the well layers of GaInNAs, emitting light in a wavelength band of 700 to 1650 nm by forming each of the well layers of GaInAsP, emitting light in a wavelength band of 800 to 1400 nm by forming each of the well layers of GaInAs, and emitting light in a wavelength band of 600 to 1650 nm by forming each of well layers of AlGaInAs, respectively.